More pictures would help. What does the sectional face?
More or larger art may help....and some more color. The “temporary” rug is the only interesting contrast in the space. MIL is right to suggest it. Maybe put that rug in front of the sectional.

I don’t like the rug it is just one more blah thing in the space . It also makes no sense where it is.I like the piano but moved over to get it out of the corner with the guitar hung above it at an angle. Take the little table away . Another nice large piece of art in the DR. I would look at getting a wardrobe for coats and shoes it would also give some separation from the entry to the DR and take away that clutter then you could move the credenza a bit closer to the door with a nice mirror above it or another piece of art. Use the colors from the art for the pillows on the sectional. If you MIL wants a rug she needs to find a smaller one with some color Greek blue would be nice.

I am confused by the brown carpet. Is that a huge rug or carpet under the sofa? I am a minimalist too but a nice modern console or accessorized bookshelves on the back of the sofa might make the sofa look a little less pushed off to the side on that brown carpet. Of course photos can be deceiving so it’s hard to tell without dimensions. Large art also a great idea.

Oh, I see now with my bad eyes :) that is the rug. I would ditch it for a better looking and more appropriate sized rug for the sectional to sit on.

Also, float your dining table and chairs instead of having them stuck in the corner. After you get an armoire for coats, move that credenza near the dining area.

Small thing, but what stood out to me is the color of the tablecloth. I'd like to see a dark neutral or a color picked up from the rug. If that pinky-red is in the rug, it doesn't show up well in the photo.

Are you able to move the light fixtures in the ceiling? I agree with other posters that you have beautiful pieces already to work with. If you can draw out the floor plan on graph paper showing all the dimensions it would help. I am wondering if you can move the dining table out of where it is, build a wall unit on that wall by the front door that would house a small coat closet, shelves and place for the TV. I would them rotate the sectional so it then faces the front door when you enter the apartment so it is more welcoming than walking into the back of the sectional and looking at the long line of smaller pieces of furniture that you have along that wall. You need some height and color in the room, especially if you live in Greece! I love the artwork you have on the far wall and I would pull from that if that is your preferred color palate? If not find a piece of art or area rug that is and start with that. If you are able to rotate the layout of the living room, your dining table can then go into the space directly across from the front door where the piano is now... I'm guessing your kitchen is just on the other side of that wall, so this might be more convenient anyway?